The invention will be described in relation with PDP but may be applicable to other types of displays as mentioned above.
As well known, a plasma display panel is constituted by two insulating plates sealed together to form a space filled with gas. Ribs are provided inside the space to form a matrix array of discharge cells which could only be “ON” or “OFF”. Also, unlike other displays such as CRT (Color ray tube) or LCD (Liquid Crystal Display) in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame. These light pulses are known as sustain pulses. The time-modulation will be integrated by the eye over a period corresponding to the eye time response.
To achieve a good picture quality, contrast is of paramount importance. However, on plasma display panels (PDPs), contrast values are inferior to those achieved for CRTs due, at least, to the following reasons:
In a PDP, it is common to use a certain amount of priming operations per frame of video picture. This priming process which makes a pre-excitation of the plasma cell is required to prepare the cells for homogeneous writing of each sub-period of the frame called “sub-fields”. In known addressing modes, two types of priming pulses can be distinguished hard-priming pulses (square form pulses, with very fast increasing slope) which are used once per frame period and soft priming pulses (triangular form pulses, with slow increasing slope) which are presently used once per sub-field. Actually, the second type of priming is used in almost every panel type. The priming process has the negative effect that a panel background light is generated. The hard priming operation creates important background luminance which reduces achievable contrast factor. The soft priming operation is used for each sub-field. It creates less background luminance per operation, but because soft priming is in general used many times per frame, this will increase the background and the total result may be worse. The same problem will arise, if more sub-fields are used in each frame since the number of priming operations is commonly linked to the number of sub-fields.
In addition, the panel efficacy (lumen/watt) is limited, and for a given power consumption of the PDP, only a limited luminance can be performed on the screen depending on the picture content.
To overcome the drawback of reduced contrast, it has been proposed, in PCT patent application No. WO01/56003 in the name of THOMSON Licensing S. A., to increase contrast of a PDP by the use of “self-priming” and “refreshing sub-fields”. Self priming sub-fields reduce or eliminate the need for priming, thus making dark areas darker, while refreshing sub-fields can be addressed faster. In practice, the number of refreshing sub-fields in a frame period is higher than the number of the self-priming sub-fields. Therefore, the total addressing time can be reduced with this new technique.
Faster addressing leaves more time for sustain pulses, thus allowing bright areas that are brighter. This is especially true for PDP monitors connected to 75 Hz multimedia sources, because in order to have an acceptable number of sub-fields, picture power is normally limited for 75 Hz sources. In 50 Hz and 60 Hz modes, where picture power is normally limited by the power electronics, a reduced addressing time may be alternatively used for increasing the number of sub-fields and thus improving picture quality.
In fact, the concept described in the above PCT patent application works well in case of full-white pictures having a limited maximal white value (for example 100 cd/m2 with around 150 sustain pulses). In that case, since the soft-priming light emission is below 0,1 cd/m2, the contrast ratio is beyond 1000:1 in dark room. Nevertheless, experiments have shown that, when the number of sustain pulses grows, the biggest sub-fields will suffer from response fidelity problems. There are many reasons for that. For example:
The sub-fields are far away from the priming pulse located at the beginning of the frame and therefore more sensitive to response fidelity problems.
Such sub-fields contain more energy, which also generate more heating of the cell. Since the response fidelity problem increases with the temperature, such sub-fields generate more problems during an increasing of the overall luminance.
In addition, when the number of sustain pulses of a given sub-field increases too much, its inertia increases at the same time and response fidelity problems are encountered.